WHY has Oxford University Press brought out a revised edition of RichardEllmann's ''JamesJoyce'' nearly a quarter-century after this definitive biography first appeared? Because 1982 is the centennial year of Joyce's birth, it seemed a fitting occasion to add the nearly 100 pages worth of information, as well as 80 new illustrations (some of which appear for the first time), that have accumulated in Professor Ellmann's hands since his book first was published in 1959.

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And what is the nature of all the new information that has been added to the text and notes? One need only scan the two pages of ''Acknowledgments 1982 Edition'' to get the idea. Gratitude goes to the curators of new collections, such as the Ezra Pound papers at Yale's Beinecke Library, or the Harriet Shaw Weaver (Joyce's patron) collection in the British Library, that have become available since 1959; and to Dr. Melissa Banta, ''for access to some valuable Joyce letters''; Samuel Beckett, ''for Joyce's limerick on Murphy''; the late Frank Budgen, ''for ampler accounts of Joyce's relationship with Marthe Fleischmann and of his own quarrel with Joyce''; David DuVivier, ''for help on two points of French philology''; Dr. T.B. Lyons, ''for the autopsy report on Joyce''; William McGuire, ''for valuable suggestions about Dr. C.G. Jung's treatment of Lucia Joyce''; Mrs. Vera Russell, ''for her recollection of Joyce's attitude toward childbirth''; and so forth.

In short, the new material is composed of an amplitude of fairly minute details, which is entirely appropriate and desirable, considering the obsessive sort of attachment that Joyce's art inspires.

But for a Joyce-nut who somehow never got around to reading straight through the Ellmann biography, the publication of the revised edition provides an opportunity to fill a gaping hole in one's education. And the effect of this experience is fairly stunning, not alone because of the remarkable wealth of details that the author has gathered up and artfully pieced together. What also strikes a reader is the number of those details that wound up in Joyce's fiction, or, to put it the other way around, the degree to which Joyce's art was grounded in actuality. As Professor Ellmann writes, Joyce ''was never a creator ex nihilo; he recomposed what he remembered, and he remembered most of what he had seen or had heard other people remember.''

Most readers have known this, at least up to a point, and so have even taken the liberty of basing their image of Joyce as an individual on his fiction. The trouble is, we never went quite far enough, but instead confined ourselves to allowing the character of Stephen Daedalus to stand for his creator, which was fair enough so far as the young Joyce was concerned, but which was hardly adequate to a portrait of the artist as an older man.

The effect of this error seems to have contributed to a misunderstanding of Joyce's masterpiece ''Ulysses'' - specifically, a tendency to look down upon the character of Leopold Bloom, to see him as a parody of his Greek predecessor, or to dismiss him as a failed father figure of Stephen.

Professor Ellmann argues eloquently against this belittling of Bloom - and, incidentally, if one considers how Bloom's stature has grown in various critics' regard since Ellmann's book first appeared, one can see that his eloquence has had an effect. But his book does something even more striking in defense of Bloom as a hero of modern literature. It shows that for all the correspondences that exists between Stephen Daedalus and the young, rebelious Joyce, there are even more between Leopold Bloom and both the mature Joyce himself and various men he happened to regard with respect.

Moreover, these parallels lend stature to Bloom in an additional way. As bumbling and inept as his progress around Dublin may seem, it is altogether stately compared with Joyce's own erratic passage through life, considering the windmills of debt, drunkenness and disaster that were forever toppling him off his wayward course. In the light of Professor Ellmann's biography, one almost has to accept Joyce's defense of himself, when his brother Stanislaus attacked him for ''intemperance'' while he was writing ''Ulysses,'' as ''the foolish author of a wise book.'' Oftentimes James Joyce was worse than foolish. But as Richard Ellmann impresses upon us, especially in his dazzling chapters on ''The Dead,'' ''Portrait of the Artist'' and ''Ulysses,'' Joyce in his art nearly always transcended himself.